Frequency of extreme El Niños to double as globe warms

Global-warming’s impact on the tropical Pacific Ocean has been hotly contested, but an analysis now suggests that powerful warming events in the eastern equatorial region, known as El Niños, are likely to double as greenhouse-gas emissions rise this century.

The study, published today in Nature Climate Change, focuses only on extreme events such as the 1997-1998 El Niño, which affected weather across the globe. Climate models have long differed regarding global-warming’s precise impact on El Niño and its sister effect, La Niña, which cools the same area. But the current analysis finds agreement when it comes to major rainfall trends caused by the most intense El Niños.

“The present day’s moderate El Niño becomes an extreme event in the future,” says lead author Wenju Cai, a climate modeller at the Commonwealth Scientific and Industrial Research Organization in Aspendale, Australia.

The key, Cai says, was to essentially redefine extreme El Niños and focus on the weather rather than the condition of the ocean itself. Instead of using the standard definition, which focuses on the difference in ocean surface temperatures between the eastern and western equatorial Pacific, the researchers focused on rainfall on the eastern side of the basin.

The team identified 20 climate models — half of those available — that were capable of simulating extreme rainfall. They then used the models to compare the occurrence of extreme El Niños in a control period, 1891 to 1990, versus a warmer period extending from 1991 to 2090. Although the total number of El Niño events decreased, 17 out of the 20 models predicted more major El Niño events, with the average frequency increasing from once every 20 years to once per decade.

“I’ve been at this long enough to know that you have to look at everything five times before you are entirely convinced, but I like the paper,” says David Neelin, a climate scientist who studies ocean?atmosphere interactions at the University of California, Los Angeles. “It’s a very promising analysis.”

The models suggest that the eastern tropical Pacific is likely to warm more than the west, and that extra warmth effectively primes the pump for major storms. As such, even smaller El Niño events — as defined by the temperature gradient across the basin, with warming in the east — can have larger effects on rainfall.

The most immediate impact would be on neighboring countries such as Peru and Ecuador, but Cai says that major El Niños could affect the weather in far-flung locales, from southeast Asia to the American Southwest. “It’s really a massive reorganization of atmospheric circulation,” he says, noting that the 1997-1998 El Niño inflicted tens of billions of dollars in damage and killed thousands of people around the globe.

More research will be needed to assess the global consequences, but the study nonetheless advances El Niño research in a way that is more relevant to society, says Shang-Ping Xie, a climate modeller at the Scripps Institution of Oceanography in La Jolla, California. “It kind of steals attention away from the traditional definition of El Niño and focuses on a more useful and relevant variable like rainfall,” he says.